Liquid-liquid contactor



United States Patent O1 tice 3,489,526 Patented Jan. 13, 1970 U.S. Cl. 23-267 13 Claims A mixer-settler unit comprising separate mixer and settler vessels with gravity ow from the former to the latter. The mixer vessel comprises a lower and an upper compartment and a draught tube leading from the former to the latter. In the lower compartment the mixing and pumping functions are separated and there is provided at least one mixing impeller and at least one pumping impeller, the latter lifting the liquid from the lower compartment of the mixer vessel from where it flows by gravity to the settler vessel. The settler vessel comprises a diffuser chamber receiving the liquid arriving from the mixer, and a settler chamber.

The present invention concerns liquid-liquid contactors of the mixer-settler type.

A large variety of mixer-settlers is described in literature. As a rule a number of mixer-settler units consisting each of one mixer vessel and one settler vessel, are combined in series in a counter-current multistage manner to form a mixer-settler battery through which the liquids to be contacted proceed in counter-current flow.

The various mixer-settlers may be classified into three types according to the manner by which the flow of the liquid through the apparatus is brought about. In one type, generally referred to as the gravity flow type the driving force for the liquid flow is the head between the inlet and outlet. For the proper functioning of such a device the hydraulic communications between the vessels must be sufficiently large in order to minimize the flow resistance. However, by increasing the cross-sectional area of the apertures between the vessels the danger of back-mixing is equally increased. Consequently in designing a gravity flow type mixer-settler one is faced with two incompatible requirements of increasing on the one hand the cross-sectional area of the apertures between the vessels in order to reduce the flow resistance, and of decreasing on the other hand the same diameter in order to reduce the back-mixing. These incompatible requirements impose serious limitations on the design of -apparatus of this type.

In another known type of mixer-settlers the flow of the liquid is brought about by pumping between stages. However, the introduction of interstage pumping means complicates the apparatus and renders it bulky. Moreover, the use of mechanical pumps for this purpose has the drawback of introducing additional moving parts which increases the power consumption and maintenance costs. In order to overcome this drawback it was suggested to use for the interstage pumping an airlift. This, however, has the disadvantage of causing volatilization of the solvents so that scrubbing of the air for the recovery of the solvents becomes necessary, both for reasons of economy and security. Also the introduction of air may cause undesired oxidation.

In a third type of mixer-settlers, generally referred to as the pump-mixing type, the pumping is combined with the mixing in the mixer. Here again it has been suggested to use an airlift by introducing air at the bottom of each mixer through a nozzle over which is placed a vertical lift tube. The disadvantages inherent in this method are the same as those mentioned above in connection with the use of an `airlift for interstage pumping. In addition, the introduction of air during the mixing may cause the formation of fine emulsions which are difficult to separate.

Therefore, most commonly the pump-mixing in mixersettlers of the pump-mix type is brought about mechanically in that the impeller of each mixer vessel acts as a pump for advancing the liquid to the associated settler vessel. The main disadvantage of this method is the complete coupling of the mixing and pumping functions. In

, consequence, the mixing has to have the same orientation as the pumping and it is, for example, impossible t0 combine an upward pumping with a downward thrust for mixing. Moreover, where in such an apparatus it is desired to vary the rate-of flow the impeller speed has to be varied accordingly. This, however, affects the mixing. There thus exists a direct relationship lbetween the rate of flow and the mixing which inmany instances is highly inefficient since for efficient operation it may be necessary to increase the rate of flow and to decrease at the same time the mixing intensity or vice versa.

In all the three known types of mixer-settlers there exists a direct hydraulic communication between the mixer and settler vessels. In consequence the mixed liquid phase from each mixer vessel is introduced Within the body of liquid present in the associated settler vessels. This in turn causes a turbulence inside the settler vessel that counteracts the phase separation in that vessel, which is manifested in an undesired increase of the turbid dispersed phase located between the clear upper and lower phases, with the result that special measures have to be taken in order to prevent the entrainment of one phase in the other. As a rule such measures consist in so dimensioning the settler vessel that each phase will always be of sufficient depth in order to prevent entrainment.

The above disadvantage inherent in hydraulic communication between mixer and settler vessels is overcome in a mixer-settler described in Chem. and Process Engineering, January 1963 and November 1963. This mixersettler is characterized by a hydraulic separation of the mixer and settler vessels from each other so that the transfer of the mixed liquid phase from each mixer to the associated settler vessel is by gravity ow. This is achieved by providing `on the top of each mixer vessel a tray communicating with the actual mixer chamber by a draught tube and by locating the mixer-pumping impeller inside this tube so that the mixed phase is drawn upon onto the tray. From that tray the mixed phase is discharged by free gravity flow into the associated settler vessel from where the phases are withdrawn separately and fed to their respective mixers, again by gravity flow. While in this mixer-settler the desired hydraulic independence of mixer and settler is achieved there still remains the fundamental deficiency that the same impeller is used both for mixing and for pumping.

It is the object of the present invention to provide an v improved liquid-liquid contactor of the mixer-settler type free of the disadvantages of the foregoing types.

1n the following specification and claims the term free flow will be used for designating the discharge of liquid by gravity from one vessel into another without hydraulic communication between the liquid bodies in the two vessels.

The invention consists in a liquid-liquid contactor of the mixer-settler type including a mixer vessel comprising a lower compartment and an upper compartment separated from each other by a horizontal partition merging in its centre into a depending draught tube, at least one mixer impeller in the lower compartment in the region below said draught tube, at least one pumping impeller within said draught tube adapted to lift a mixed liquid phase from the lower into the upper compartment, means for the free-flow discharge of the mixed liquid phase from the upper compartment and means for the separate introduction of two liquids into the lower compartment; a settler vessel comprising a diffuser chamber adapted to receive said mixed liquid phase arriving by free flow from the mixer vessel, a settler chamber surrounding the diffuser chamber, peripheral collector means in said settler chamber for the overflow of the light phase and means near the bottom of said settler chamber for the free-flow discharge of the heavy phase; and means leading from the upper compartment of the mixer vessel to the settler vessel adapted for the freeflow discharge of a mixed liquid phase from the former to the latter.

It is thus seen that in accordance with the invention the functions of mixing and pumping in the mixer vessel are performed separately. As a result of this lfunctional separation it is possible to design the mixing impeller(s) for any desired mixing intensity and the pumping impeller(s) for any desired rate of flow. It is thus possible to combine a high mixing intensity with a low rate of flow or vice versa, a low mixing intensity with a high rate of flow. It is furthermore possible to use a downthrust mixer impeller without interfering with the pump ing efficiency. As mentioned before, this is impossible in cases where the same impeller is used for mixing and for pumping where the thrust is in the direction of the pumping.

The mixing and pumping impellers may be driven by the same or separate driving means. For example, the

v mixing impellers may be located on the bottom of the lower compartment and be driven mechanicaly or magnetically from below, while the shaft of the pumping impeller(s) may either depend into the draught tube from above or penetrate into it from below the mixer. Various modifications are possible in particular as regards the location and the driving of the mixing impeller(s).

In a preferred embodiment of the invention the mixer and pumping impellers are each keyed on a shaft depending from overhead driving means. In accordance with this preferred embodiment the shaft may either be common to both the mixer and pumping impellers or, alternatively, separate shafts may be provided. In the latter case the shaft for the pumping impeller(s) may be hollow and loosely envelope the shaft for the mixer irnpeller, possibly with interposition of suitable bearings. If desired the two shafts may be coaxial.

Where in accordance with the invention separate shafts are provided for the mixer and pumping impellers, different speeds of revolution may be applied which increases the versatility of the mixer. The two shafts may either be linked to different driving means or alternatively to the same driving means through the intermediary of separate speed transformer means.

The upper chamber in a mixer of a liquid-liquid contactor according to the invention will as a rule be much 'shallower than the lower one and may be of a tray-like design. In addition to its primary function of enabling the free flow of the liquid from the mixer to the associated settler, the upper chamber has the additional beneficiary effect of enabling a preliminary coalescence which may contribute towards simplification of design and reductio-i in size of the associated settler vessel.

The diffuser of the settler vessel is constituted by liquid-permeable partitions which may be perforated, porous, slotted or shutter-like and which divide the settler vessel into an inner diffuser chamber and an outer surrounding settler chamber. These partitions function as coalescence inducers. The use of such means for the induction of the coalescence is known from conventional settler vessels but in accordance with the invention Athe diffuser may as a rule be of simpler design than in conventional mixer-settlers because of the preliminary coalescence taking place in the upper compartment of the mixer. Any increase of the flow resistance due to the diffuser is of no consequence because of the hydraulic independence of the mixer and settler vessels.

The principle of free-flow of the mixed liquid phase from the mixer vessel to the settler vessel enables delivery of the mixed liquid phase at any desired point across the latter. For example it is possible to locate the diffuser chamber in the centre of the settler chamber and to deliver the mixed liquid phase into thevcentre of the former. Such an arrangement in combination with a peripheral peripheral overflow channel for the light phase which occupies the entire settler circumference and possible also a peripheral withdrawal of the heavy phase gives equal flow distribution throughout the settler and eliminates the turbulence due to directional flow. Peripheral overflow also reduces to a minimum the fluid velocity at the overow point. The low fluid velocity and elimination of turbulence permit operation with a minimum depth of clear liquid; the secondary break i.e. coalescence of finely dispersed droplets, can take place unhindered by turbulence or high localized uid velocities. Such a regime ermits operation with a minimum depth of clear liquor i.e. the settler capacity is increased.

In liquid-liquid extraction there exists also the problem of operation at high phase ratios. Normally such operation is possible only with recycle of one phase in order to improve coalescence. The reduction of fluid velocity and turbulence in the proposed contactor permits operation at any required ratio.

In a preferred embodiment of a liquid-liquid contactor according7 to the invention there is provided a by-pass connecting the lower compartment of a mixer vessel with the settler vessel below the liquid level in either vessel under normal operational conditions, which by-pass is fitted with pressure-sensitive control means adapted to close and remain closed when the hydrostatic pressure in the mixer vessel corresponds to opeartional conditions, and to open when the hydrostatic pressure in that vessel exceeds the value corresponding to normal operational conditions.

Owing to the provision of the above by-pass the flow of liquids through a particular contactor forming part of a battery may continue even if the pumping impeller of that unit breaks down. Upon such breakdown no liquid will be pumped up into the upper mixer comparement while the introduction of liquid into the lower compartment will continue. In consequence the hydrosttic pressure in the lower compartment will rise and eventually the control means of the by-pass will open and the excess liquid `be discharged into the associated settler. When upon this discharge the liquid level in the mixer returns to normal the control means of the bypass will close again. Where the breakdown of the pumping impeller is not accompanied by a breakdown of the mixer impeller the effect of the by-pass is that the particular contactor concerned is temporarily converted into a conventional gravtiy-flow unit with direct hydraulic communication 'between the mixer and settler vessels. Where, on the other hand, the breakdown of the pumping impeller is accompanied by a breakdown of the mixer impeller the effect of the by-pass is a temporary complete cut-out of the particular contactor concerned. In either case the remainding contactors of the battery will continue to function normally, the consequence being a temporary reduction of the total extraction efficiency of the battery which in many industrial operations is preferable to a total interruption of operation.

It follows from the above that a liquid-liquid contactor of the mixer-settler type according to the invention embodies in combination in an original and most eicient manner the following features:

(a) Free flow from and into each mixer and settler vessel with no hydraulic communications between the vessels and equal flow distribution inside the settler vessel. There is thus no danger of back mixing and the turbulence in the settler vessels is greatly reduced. Consequently each phase may be shallow without danger of entrainment which means that the size of the settler vessels may be considerably smaller than the size of a corresponding vessel in a conventional apparatus designed for the same through-flow.

(b) The use of a pump-mix unit avoiding thereby the introduction of interstage vessels and/ or pumps.

(c) The separation of the mixing and pumping functions so that each may =be performed independently according to its own requirements.

The free flow conditions obtained in a liquid-liquid contactor according to the invention lead to a further great advantage in that they ensure a far-reaching similarity between model and prototype. Such a similarity is, as is well known, dicult if at all possible to achieve in conventional equipment of this kind. Consequently in the design of a liquid-liquid contactor according to the invention the upscaling does not constitute a serious problem as it does in conventional mixer-settlers.

The invention further provides for use in a liquid-liquid contactor as hereinbefore defined, a mixer vessel which comprises a lower compartment and an upper compartment separated from each other by a horizontal partition merging in its centre into a depending draught tube, at least one mixer impeller in the lower compartment in the region below said draught tube, at least one pumping impeller within said draught tube adapted to lift a mixed liquid phase from the lower into the upper compartment, means for the free-fiow discharge of the mixed liquid phase from the upper compartment and means for the separate introduction of two liquids into the lower cornpartments.

The invention also provides for use in a liquid-liquid contactor as hereinbefore defined a settler vessel which comprises a ditfuser chamber adapted to receive said mixed liquid phase arriving by free flow from the mixer vessel, a settler chamber surrounding the dilfuser chamber, peripheral collector means in said settler chamber for the overflow of the light phase and means near the bottom of said settler chamber for the free-ow discharge of the heavy phase.

Finally the invention also provides liquid-liquid contactor batteries wherein a number of liquid-liquid contactors as hereinbefore dened are combined in a multistage countercurrent manner with free-flow interstage liquid transfer.

The invention is illustrated, by way of example only, in the accompanying drawings in which:

FIG. 1 is a diagrammatic section of a mixer for a liquid-liquid contactor according to the invention;

FIG. 2 is a diagrammatic section of one embodiment of a liquid-liquid contactor according to the invention;

FIG. 3 is a plan view of the contactor according to FIG. 2 with the cover of the settler removed;

FIG. 4 is a plan view of another embodiment of a liquid-liquid contactor according to the invention; and

FIG. 5 is a plan view of the contactor according to FIG. 4.

The mixer illustrated in FIG. 1 comprises a lower, mixing compartment 1 and an upper, substantially at compartment 2 separated from each other Iby a horizontal partition 3 which in its centre merges into a depending draught tube 4. The lower compartment 1 comprises ports 5 and `6 for the introduction of the two liquids to be contacted in the mixer. The level of the liquid body inside compartment 1 under normal operational condi- 6 tions is indicated by the horizontal hyphenated line and as is seen in FIG. 1 ports 5 and 6 are above that level. The upper compartment 2 comprises a port 7 for the free-110W discharge of liquid therefrom.

A shaft 8 connected to an overhead driving motor (not shown) extends through tube 4. On the lower end of shaft 8 outside of tube 4 is keyed a mixer impeller 11. Also keyed on the same shaft and located within tube 4 is a pumping impeller 12 situated between straightening vanes 9 and 10 forming an integral part of the pump.

During operation the two liquids that are to be contacted in the mixer are introduced into compartment 1 through ports 5 and 6 and are intimately mixed inside the compartment by means of the mixer impeller 11. From compartment 1 the mixed liquid phase is lifted by the submerged pumping impeller 12 to the upper chamber 2 from where it is discharged through port 7. The design of each of the impellers 11 and 12 is such as to meet its requirements. Thus the mixer impeller 11 is designed for the required mixing intensity. If desired impeller 11 may be exchangeable or adjustable in order to meet varying requirements. Likewise the pumping impeller 12 is designed to meet the desired flow requirements which may 'be selected in complete independence of the mixing requirements. Also in this case the impeller may be adjustable or exchangeable. It is thus possible to combine a high mixing intensity with a low rate of tiow or vice versa.

In the embodiment of the mixer illustrated in FIG. l the mixer impeller 11 and the pumping impeller 12 are keyed on a common shaft. If desired, this design may be modified by having the impellers 11 and 12 keyed on separate shafts, the one for the pumping impeller 12 being hollow and loosely enveloping the one of the mixing impeller 11, if desired with the interposition of suitable bearings. In this manner the two impellers may be revolved at different speeds which is a further contribution towards the versatility of the mixer according to the invention.

A further modification consists in using two or more pumping impellers located one above the other Within the draught tube 4.

The liquid-liquid contactor according to the invention illustrated in FIG. 2 comprises a mixer 15 of the kind illustrated in FIG. 1 and a settler 16. The upper compartment 18 of the mixer is connected by a chute 19 to a collector tray 20 of the settler, which tray comprises a discharge opening 21 located above a diffuser 22. The latter is a chamber of upright prismatic shape separated from the surrounding main portion of the settler by slotted partitions. The chamber may also be circular.

The lower portion of mixer compartment 17 communicates with the lower portion of settler 16 by means of a horizontal by-pass pipe 23 tted at the end opening into the settler with a pressure sensitive shutting device 24 adapted to close and remain closed when the hydrostatic pressure in compartment 17 corresponds to operational conditions. If for some reason the liquid level inside compartment 17 rises so that the hydrostatic pressure exerted on the device 24 is increased, the latter will open and liquid will flow from compartment 17 into settler 16.

Settler 16 further comprises an upper peripheral duct 25 for the collection and free-flow discharge of the upper, lighter phase. Near its -bottom the settler 16 comprises a luted port 26 for the discharge of the heavy phase. From above settler 16 is covered by a cover 27 inserted into a liquid seal 28. For operation a number of units according to FIGS. 2 and 3 are combined into a multistage countercurrent battery.

During operation the two liquids to be contacted are fed into each mixer at a properly selected rate. The mixer phase is pumped from each compartment 17 into the upper compartment 18 where a preliminary coalescence occurs and from where the liquid flows freely through chute 19 onto the collector plate 20 from where it is discharged by free fall through opening 21 into the diffuser 22. From the latter the liquid diffuses through the partitions into the surrounding main portion of the settler, by which diffusion further coalescence of the phases is induced. In the main portion of the settler vessel phase separation occurs and the upper phase overflows into duct 2S from where it is discharged while the lower phase is discharged from port 26. Each of the two phases is conducted by free flow into another coordinated mixer in accordance with the counter-flow scheme.

If in operation the pumping impeller of the mixer ceases to operate, e.g. because of a breakdown of the driving motor, no liquid will be lifted into the upper compartment 18. Assuming the remaining contactors of the same battery to continue to operate, further quantities of liquid will be introduced into compartment 17 so that the liquid level in that compartment will gradually rise. Eventually the hydrostatic pressure exerted on the shutting device '24 will have increased to such an extent that this device will open and some of the liquid inside mixer compartment 17 will be discharged into settler 16 through bypass 23. In this manner the flow of the liquid through the contactor unit is ensured and the ybattery as a whole may continue to operate.

The embodiment of a liquid-liquid contacter illustrated in FIGS. 4 and 5 is basically similar to that according to FIGS. 2 and 3 and differs therefrom merely by the fact that the mixer vessel is located within the settler.

As shown the mixer vessel 30 is identical to the one according to FIG. 1, compris-ing a mixing compartment 31 and an upper compartment 32, which latter comprises discharge openings 33. Vessel 30 is located within diffuser 34 of settler vessel 35. Settler vessel 35 comprises a peripheral duct 36 for the collection and free-flow discharge of the upper, lighter phase, and near its bottom a luted port 37 for the discharge of the heavy phase.

Ducts 38 and 39 -lead `into the mixing compartment 31 and serve for the introduction of the two phases to be contacted.

Vessel 30 comprises a spout 40 leading into diffuser 34 and fitted with a pressure sensitive shutter 41. The Ifunction of spout 40 and shutter 41 is completely analogous to by-pass pipe 23 Iand shutter 24 of the embodiment of FIGS. 2 `and 3.

The functioning of this device is fully identical to the Ifunctioning of the device according to FIGS. 2 and 3 and need therefore not be described.

What we claim is:

1. A liquid-liquid contactor of the mixer-settler type including a mixer vessel comprising a lower compartment and an upper compartment separated from each other by 4a horizontal partition merging in its center into a depending draught tube, means for separately introducing at least two liquids into the lower compartment, one of la heavier phase than that of the other liquid, `at least one mixer impeller in the lower compartment in the region below said draught tube, at least one pumping impeller within said draught tube adapted to lift the mixed liquid phase from the lower into the upper compartment, and means for the substantially horizontal free-flowing lengthy discharge of the mixed phase from the upper compartment; a settler Ivessel comprising a diffuser chamber adapted to receive said mixed liquid phase arriving by said substantially horizontal lengthy free-flow from the mixer vessel, a settler chamber surrounding the diffuser chamber, a substantially vertical liquid-permeable partition between said diffuser chamber and said settler chamber to permit radially outward flow of the liquid into the settler chamber, peripheral collector means in said settler chamber for the overflow of the light phase fand means near the bottom of said settler :chamber for the Ifree-flow discharge of the heavy phase.

2. A contactor according to claim 1, wherein the mixer land pumping impellers are each keyed on Ia shaft depending from overhead driving means.

3. A contactor according to claim 2, wherein the pltllgping and mixing impellers are keyed on a common s t.

4. A contactor according to claim 2, wherein the pumping `and mixing impellers are keyed on separate shafts.

5. A contactor according to claim 4, wherein the shaft of the pumping impellers is hollow and envelops loosely the shaft of the mixing impeller.

6. A contacter according to claim y1, comprising a by-pass connecting the lower mixer `compartment with the settler below the levels which the liquid in both vessels assume under flow conditions in both vessels corresponding to the design values, which by-pass is fitted with pressure-sensitive How control means adapted to close and remain closed when the hydrostatic pressure in the mixer compartment corresponds to normal operational conditions, -and to open when the hydrostatic pressure in the mixer compartment exceeds the value corresponding to normal operational conditions.

7. For use in a liquid-liquid contactor according to claim 1, =a mixer vessel comprising a lower compartment and an upper compartment separated `from each other by a horizontal partition merging in its center into a depending draught tube, means for separately introducing lat least two liquids into the lower compartment, one of a heavier phase than that of the other liquid, lat least one mixer impeller in the lower compartment in the region below said draught tube, at least one pumping impeller within said draught tube yadapted to lift mixed liquid phase from the lower into the upper compartment and means for the substantially horizontal free-flowing lengthy discharge of the mixed phase from the upper compartment.

8. A mixer vessel according to lclaim 7, wherein the mixer and pumping impellers are each keyed on a shaft depending from overhead driving means.

9. A mixer vessel according to claim 8, wherein the 'pumping Iand mixing impellers are keyed on a common shaft.

10. A mixer vessel according to claim 8, wherein the pumping and mixing impellers Iare keyed on separate shafts.

l1'1. A mixer vessel according to claim y10, wherein the `shaft of the pumping impellers is hollow and envelops loosely the shaft of the mixing Iimpeller.

112. For use in a liquid-liquid contacter according to yclaim 1, Ia settler vessel comprising a diffuser chamber adapted to receive said mixed liquid phase arriving by said substantially horizont-al lengthly free-flow from the mixer vessel, a settler chamber surrounding the diffuser chamber, a substantially Vertical liquid-permeable partition between said diffuser chamber and said settler chamber to permit radially youtward flow of the liquid into said settler chamber, peripheral collector means in said settler chamber for the overflow of the light phase and means near the bottom of said settler chamber for the free-flow discharge of the heavy phase.

13. A liquid-liquid contacter apparatus comprising a battery of liquid-liquid contactors according to claim 1 in la multistage counter-current manner with free-flow interstage liquid transfer.

References Cited UNITED STATES PATENTS 2,164,189 6/1939 Hund 23--270.5 X 2,201,550 5/1940 Van Dijck 23-270.5 X 2,405,158 8/1946 Mensing 23-270.5 2,408,721 10/ 1946 Altsheler 23--267 X 2,594,675 4/ 1952 Norell 23--270.5 2,701,753 2/1955 Eisenlohr 23-310 2,710,250 6/1955 Andrews 23-267 (Other references on following page) 9 UNITED STATES PATENTS 10/1956 Hart 23--270.5 X 12/1964 Meissner 23-267 FOREIGN PATENTS 1 0 NORMAN YUDKOFF, Primary Examiner S. I. EMERY, Assistant Examiner U.S. C1. X.R. 

